The present invention relates to a connection between a needle bar and connecting rod in a needle bar drive in zig-zag sewing machines comprising the crank mounted at the end of the main shaft of the sewing machine, the connecting rod connected at one end to the crank and at the other end to a pin of a needle bar block and a control rod suitable to impart to the needle bar transverse movements in the fabric feed direction during which the needle bar longitudinal movements remain parallel.
In the drive when the needle bar moves as described above for zig-zag sewing, the needle bar block pin slides inside the head of the connecting rod. Either for parallelism errors between the members of the kinematic chain driving the needle bar or for the use of particular devices providing the transverse displacements of the needle bar for obtaining patterns or zig-zag stitches, jamming of the block pin may occur inside the big end of the connecting rod.
In fact, in sewing machines where the transverse movements of the needle bar are derived from a gradually variable profile of a cam, the relative slipping between the block pin and the big end of the connecting rod lasts for a considerable period of the cycle and is accompanied by a relative rolling between the two members of the coupling. On the contrary, in the electronically operated sewing machines where the needle bar transverse displacements are controlled by linear or stepping motors, actuated by current pulses, having a very short response time, the relative slipping between the block pin and the big end of the connecting rod may take place at a very high speed without the relative rolling between the members.
Jamming phenomenons, caused by unavoidable parallelism errors and by the load of an inertial kind on the coupling, reveal harmful repeating of the working cycles. An object of the present invention is to eliminate this possible inconvenience.
The technical problem to be solved in order to obtain the object mentioned was to create a particular coupling suitable to eliminate the described drawback.
The solution of the technical problem is characterized by the fact that the connection between the needle bar and the connecting rod is provided by a roller-shaped bush rotatably mounted in a seat in the fork-shaped lower portion of the connecting rod, the pin connecting the bush and the connecting rod sliding inside a bore in the bush, the bore in the bush having its axis perpendicular to the bush roller axis.